The present invention relates to mobile telephones and, in particular but not exclusively, to mobile telephones which are able to send and receive short text messages using the short message service provided by GSM Mobile Telephone Standards.
The GSM short message service (SMS) can be used by base stations and mobile units within a network to interrogate and gain information form a target mobile unit, and can be used to change operational settings of the mobile unit.
Such information could be related to the geographical position of the mobile station, or details of the current cell site in which the mobile station is operating. Some operational settings which may be modified could include details of closed user group numbers, call forwarding or barring details etc.
It is therefore desirable to provide a system in which short messages (SMs) that are intended to make operational changes or request information from a mobile unit are encoded to prevent fraudulent use.
According to one aspect of the present invention, there is provided a method of operating a mobile telephone comprising:
receiving message data including identity data, key data and instruction data from a caller, the identity data relating to the caller;
combining the identity data and key data to produce received security data;
obtaining stored security data from a memory of the telephone on the basis of the received identity data;
comparing the received security data with the stored security data; and
processing the instruction data if the received security data is equivalent to the stored security data, or rejecting the instruction data if the received and stored security data are not equivalent.